The present invention relates to an active matrix type display apparatus having an active device in each pixel and controlling display in the pixel unit by means of the active device and a driving method thereof, and particularly to an active matrix type display apparatus using an electrooptic device that varies brightness according to a current flowing therein, an active matrix type organic EL display apparatus using an organic-material electroluminescence (hereinafter described as organic EL (electroluminescence)) device as the electrooptic device, and driving methods thereof.
For example, a liquid crystal display using a liquid crystal cell as a display device of a pixel has a large number of pixels arranged in a matrix manner, and controls light intensity in each pixel according to information of an image to be displayed, to thereby effect driving for image display. The same display driving is effected by an organic EL display using an organic EL device as a display device of a pixel and the like.
Since the organic EL display is a so-called self-luminous type display using a light emitting device as the display device of a pixel, the organic EL display has advantages such as higher visibility of images, no need for a backlight, and a higher response speed as compared with the liquid crystal display. Moreover, the organic EL display differs greatly from the liquid crystal display or the like, which uses liquid crystal cells of a voltage-controlled type, in that brightness of each light emitting device is controlled by the value of a current flowing therein, that is, the organic EL device is of a current-controlled type.
As with the liquid crystal display, the organic EL display can use a passive matrix method and an active matrix method as its driving method. However, the former has a simple construction but has problems such as difficulty in realizing a large high-definition display. Thus, the active matrix method has recently been actively developed which controls a current flowing through a light emitting device within a pixel by means of an active device also disposed within the pixel, for example an insulated gate field-effect transistor (typically a thin film transistor (TFT)).
FIG. 1 shows a conventional example of a pixel circuit (circuit of a unit pixel) in an active matrix type organic EL display (For more detailed description, see U.S. Pat. No. 5,684,365 and Japanese Patent Laid-Open No. Hei 8-234683).
As is clear from FIG. 1, the pixel circuit according to the present conventional example includes: an organic EL device 101 having an anode (anode) connected to a positive power supply vdd; a TFT 102 having a drain connected to a cathode (cathode) of the organic EL device 101 and a source connected to a ground (hereinafter described as “grounded”); a capacitor 103 connected between a gate of the TFT 102 and the ground; and a TFT 104 having a drain connected to the gate of the TFT 102, a source connected to a data line 106, and a gate connected to a scanning line 105.
Since the organic EL device has a rectifying property in many cases, the organic EL device may be referred to as an OLED (Organic Light Emitting Diode). Therefore, in FIG. 1 and other figures, a symbol of a diode is used to denote the OLED. In the following description, however, a rectifying property is not necessarily required of the OLED.
The operation of the thus formed pixel circuit is as follows. First, when potential of the scanning line 105 is brought to a selected state (high level in this case) and a writing potential Vw is applied to the data line 106, the TFT 104 conducts, the capacitor 103 is charged or discharged, and thus a gate potential of the TFT 102 becomes the writing potential Vw. Next, when the potential of the scanning line 105 is brought to a non-selected state (low level in this case), the TFT 102 is electrically disconnected from the scanning line 105, while the gate potential of the TFT 102 is stably retained by the capacitor 103.
A current flowing through the TFT 102 and the OLED 101 assumes a value corresponding to a gate-to-source voltage Vgs of the TFT 102, and the OLED 101 continues emitting light at a brightness corresponding to the value of the current. The operation of selecting the scanning line 105 and transmitting to the inside of the pixel brightness data supplied to the data line 106 will hereinafter be referred to as “writing.” As described above, after the pixel circuit shown in FIG. 1, once writing of the potential Vw is done, the OLED 101 continues emitting light at a fixed brightness until next writing.
An active matrix type display apparatus (organic EL display) can be formed by arranging a large number of such pixel circuits (which may hereinafter be described simply as pixels) 111 in a matrix manner as shown in FIG. 2, and repeating writing from a voltage driving type data line driving circuit (voltage driver) 114 through data lines 115-1 to 115-m while selecting scanning lines 112-1 to 112-n sequentially by a scanning line driving circuit 113. A pixel arrangement of m columns and n rows is shown in this case. Of course, in this case, the number of data lines is m and the number of scanning lines is n.
Each light emitting device in a passive matrix type display apparatus emits light only at an instant when the light emitting device is selected, whereas a light emitting device in an active matrix type display apparatus continues emitting light even after completion of writing. Thus, the active matrix type display apparatus is advantageous especially for use as a large high-definition display in that the active matrix type display apparatus can decrease peak brightness and peak current of the light emitting device as compared with the passive matrix type display apparatus.
In an active matrix type organic EL display, a TFT (thin film field-effect transistor) formed on a glass substrate is generally used as an active device. It is well known, however, that amorphous silicon and polysilicon used to form the TFT has inferior crystallinity and inferior controllability of the conducting mechanism to single-crystal silicon, and thus the formed TFT has great variations in characteristics.
When a polysilicon TFT is formed on a relatively large glass substrate, in particular, the polysilicon TFT is generally crystallized by a laser annealing method after formation of an amorphous silicon film, in order to avoid problems such as thermal deformation of the glass substrate. However, it is difficult to irradiate the large glass substrate with uniform laser energy, and thus the crystallized state of the polysilicon is inevitably varied depending on a location within the substrate. As a result, it is not rare that the threshold value Vth of even TFTs formed on the same substrate is varied from pixel to pixel by a few hundred mV, or 1 V or more in some cases.
In that case, even when the same potential Vw is written to different pixels, for example, the threshold value Vth of the TFTs varies from pixel to pixel. This results in great variation from pixel to pixel in the current Ids flowing through the OLED (organic EL device), and hence complete deviation of the current Ids from a desired value. Therefore high picture quality cannot be expected of the display. This is true for not only variation in the threshold value Vth but also variation in carrier mobility μ and the like.
In order to remedy such a problem, the present inventor has proposed a current writing type pixel circuit shown in FIG. 3 as an example (see International Publication Number WO01/06484).
As is clear from FIG. 3, the current writing type pixel circuit includes: an OLED 121 having an anode connected to a positive power supply Vdd; an N-channel TFT 122 having a drain connected to a cathode of the OLED 121 and a source grounded; a capacitor 123 connected between a gate of the TFT 122 and the ground; a P-channel TFT 124 having a drain connected to a data line 128, and a gate connected to a scanning line 127; an N-channel TFT 125 having a drain connected to a source of the TFT 124, and a source grounded; and a P-channel TFT 126 having a drain connected to the drain of the TFT 125, a source connected to the gate of the TFT 122, and a gate connected to the scanning line 127.
The thus formed pixel circuit is crucially different from the pixel circuit shown in FIG. 1 in the following respect: in the case of the pixel circuit shown in FIG. 1, brightness data is supplied to the pixel in the form of voltage, whereas in the case of the pixel circuit shown in FIG. 3, brightness data is supplied to the pixel in the form of current.
First, when brightness data is to be written, the scanning line 127 is brought to a selected state (low level in this case), and a current Iw corresponding to the brightness data is passed through the data line 128 The current Iw flows through the TFT 124 to the TFT 125. In this case, let Vgs be a gate-to-source voltage occurring in the TFT 125. Because of a short circuit between the gate and drain of the TFT 125, the TFT 125 operates in a saturation region.
Thus, according to a well-known equation of a MOS transistor, the following holds:Iw=μ1Cox1W1/L1/2(Vgs−Vth1)2  (1)In the equation (1), Vth1 is the threshold value of the TFT 125; μl is carrier mobility; Cox1 is gate capacitance per unit area; W1 is channel width; and L1 is channel length.
Then, letting Idrv be a current flowing through the OLED 121, the current value of the current Idrv is controlled by the TFT 122 connected in series with the OLED 121 In the pixel circuit shown in FIG. 3, a gate-to-source voltage of the TFT 122 coincides with the Vgs in the equation (1), and hence, assuming that the TFT 122 operates in a saturation region,Idrv=μ2Cox2W2/L2/2(Vgs−Vth2)2  (2)
Incidentally, a condition for operation of a MOS transistor in a saturation region is generally known to be:|Vds|>|Vgs−Vt|  (3)The meanings of the parameters in the equation (2) and the equation (3) are the same as in the equation (1). Since the TFT 125 and the TFT 122 are formed adjacent to each other within a small pixel, it may be considered that actually μ1=μ2, Cox1=Cox2, and Vth1=Vth2. Then, the following is readily derived from the equation (1) and the equation (2):Idrv/Iw=(W2/W1)/(L2/L1)  (4)
Specifically, even when the values themselves of the carrier mobility μ, the gate capacitance Cox per unit area, and the threshold value Vth vary within a panel surface or from panel to panel, the current Idrv flowing through the OLED 121 is in exact proportion to the writing current Iw, and consequently light emitting brightness of the OLED 121 can be controlled accurately. In particular, when a design is made such that W2=W1 and L2=L1, for example, Idrv/Iw=1, that is, the writing current Iw and the current Idrv flowing through the OLED 121 are of the same value irrespective of variations in the TFT characteristics.
FIG. 4 is a circuit diagram showing another circuit example of a current writing type pixel circuit. The pixel circuit according to the present circuit example is in opposite relation in terms of a transistor conduction type (N channel/P channel) from the pixel circuit according to the circuit example shown in FIG. 3. Specifically, the N-channel TFTs 122 and 125 in FIG. 3 are replaced with P-channel TFTs 132 and 135, and the P-channel TFTs 124 and 126 in FIG. 3 are replaced with N-channel TFTs 134 and 136. The direction of current flow and the like are also different. However, operating principles are exactly the same.
An active matrix type organic EL display apparatus can be formed by arranging the above-described current writing type pixel circuits as shown in FIG. 3 and FIG. 4 in a matrix manner. FIG. 5 shows an example of configuration of the active matrix type organic EL display apparatus.
In FIG. 5, scanning lines 142-1 to 142-n are arranged one for each of rows of current writing type pixel circuits 141 corresponding in number with m columns×n rows and disposed in a manner of the matrix. The gate of the TFT 124 in FIG. 3 (or the gate of the TFT 134 in FIG. 4) and the gate of the TFT 126 in FIG. 3 (or the gate of the TFT 136 in FIG. 1) are connected in each pixel to the scanning line 142-1 to 142-n. The scanning lines 142-1 to 142-n are sequentially driven by a scanning line driving circuit 143.
Data lines 144-1 to 144-m are arranged one for each of the columns of the pixel circuits 141. One end of each of the data lines 144-1 to 144-m is connected to an output terminal for each column of a current driving type data line driving circuit (current driver CS) 145. The data line driving circuit 145 writes brightness data to each of the pixels through the data lines 144-1 to 144-m.
When such a circuit to which brightness data is supplied in the form of a current value, that is, a current writing type pixel circuit as shown in FIG. 3 or FIG. 4 is used as a pixel circuit, power consumption in writing the brightness data tends to be increased. The reason is as follows: the voltage writing type pixel circuit shown in FIG. 1 and the active matrix type display apparatus using the voltage writing type pixel circuit do not consume direct current in driving a data line, whereas the current writing type pixel circuit and the active matrix type display apparatus using the current writing type pixel circuit consume direct current in driving a data line.
For example, when it is assumed that, as realistic numerical values, a maximum value of writing current per data line is 100 μA, a supply voltage is 15 V, and, supposing a full-color XGA (extended graphics array) panel, the number of data lines is 1024×3(RGB)=3072, power consumption required for writing is as high as 100 μA×3072×15 V=4.6 W. To be more specific, the power consumption is lower because the writing current does not flow during a vertical blanking period, but does not differ greatly.
For lower power consumption, it suffices to simply lower the value of the writing current; in that case, however, a problem of an increase in required writing time arises. Specifically, in the current writing method, the output impedance of the current driving circuit serving as a current source is substantially infinite, and therefore the impedance of the circuit is determined by a transistor within the pixel circuit or, more specifically, the TFT 125 in the example of the pixel circuit in FIG. 3.
More specifically, when both sides of the foregoing equation (1) of the MOS transistor are differentiated with respect to the gate-to-source voltage Vgs,1/Rpix=μ1Cox1W1/L1(Vgs−Vth1)  (5)where Rpix is differential resistance of the TFT 125 as viewed from the data line 128. From the equation (1) and the equation (5), the following is obtained:Rpix=1/√(2μ1Cox1W1/L1·Iw)  (6)
As is clear from the equation (6), the differential resistance Rpix is in inverse proportion to the square root of the writing current Iw. On the other hand, a large parasitic capacitance Cdata is generally present in the data line 128. Thus, a time constant τ of the writing circuit around a steady state is substantiallyτ=Cdata×Rpix  (7)
In the current writing method, in order to stabilize the potential of the data line in a steady state, a sufficiently long writing time as compared with the time constant τ is desirable. As is clear from the equation (6) and the equation (7), however, the time constant τ becomes longer as the writing current is decreased, and since in writing black data, in particular, Iw=0, in theory, the writing is not completed within a finite time. In practice, since errors are tolerable to some extent, for example, it is possible to perform practical writing operation even within a finite writing time. However, the writing of a small current basically requires a longer writing time than the writing of a large current.
This presents a serious problem especially when low-brightness data, which means a low current value, is written, when the parasitic capacitance Cdata of the data line 128 is increased as a result of an increase in the size of the display, or in a high-definition display, in which an allowable writing time (scanning period) is shortened. The reason for its being a serious problem is that in order to complete writing operation within a predetermined period, the writing current needs to be increased, but this results in an increase in power consumption.